Base stations used in wireless telecommunication systems have the capability to receive linear polarized electromagnetic signals. These signals are then processed by a receiver at the base station and fed into a telephone network. In practice, the same antenna which receives the signals can also be used to transmit signals. Typically, the transmitted signals are at different frequencies than the received signals.
A wireless telecommunication system suffers from the problem of multi-path fading. Diversity reception is often used to overcome the problem of severe multipath fading. A diversity technique requires at least two signal paths that carry the same information but have uncorrelated multi-path fadings. Several types of diversity reception are used at base stations in the telecommunications industry including space diversity, direction diversity, polarization diversity, frequency diversity and time diversity. A space diversity system receives signals from different points in space requiring two antennas separated by a significant distance. Polarization diversity uses orthogonal polarization to provide uncorrelated paths.
As is well-known in the art, the sense or direction of linear polarization of an antenna is measured from a fixed axis and can vary, depending upon system requirements. In particular, the sense of polarization can range from vertical polarization (0 degrees) to horizontal polarization (90 degrees). Currently, the most prevalent types of linear polarization used in systems are those which use vertical/horizontal and +45.degree./-45.degree. polarization (slant 45.degree.). However, other angles of polarization can be used. If an antenna receives or transmits signals of two polarizations normally orthogonal, they are also known as dual polarized antennas.
An array of slant 45.degree. polarized radiating elements is constructed using a linear or planar array of crossed dipoles located above a ground plane. A crossed dipole is a pair of dipoles whose centers are co-located and whose axes are orthogonal. The axes of the dipoles are arranged such that they are parallel with the polarization sense required. In other words, the axis of each of the dipoles is positioned at some angle with respect to the vertical or longitudinal axis of the antenna array.
One problem associated with a crossed dipole configuration is the interaction of the electromagnetic field of each crossed dipole with the fields of the other crossed dipoles and the surrounding structures which support, house and feed the crossed dipoles. As is well known in the art, the radiated electromagnetic (EM) fields surrounding the dipoles transfer energy to each other. This mutual coupling influences the correlation of the two orthogonally polarized signals. The opposite of coupling is isolation, i.e., coupling of -30 dB is equivalent to 30 dB isolation.
Dual polarized antennas have to meet a certain port-to-port isolation specification. The typical port-to-port isolation specification is 30 dB or more. The present invention increases the port-to-port isolation of a dual polarized antenna. This isolation results from the phase-adjusted re-radiated energy that cancels with the dipole mutual coupling energy.
Generally, dual polarized antennas must meet the 30 dB isolation specification in order to be marketable. Not meeting the specification means the system integrator might have to use higher performance filters which cost more and decrease antenna gain. The present invention overcomes these concerns because it meets or exceeds the 30 dB isolation specification. Additionally, dual polarized antennas generally must achieve 10 dB cross polarization discrimination at 60 degrees in order to be marketable, i.e., must achieve 10 dB cross polarization discrimination at a position perpendicularly displaced from the central axis of the antenna and 60 degrees away from the plane intersecting that axis. The present invention provides a means to meet the 10 dB cross polarization discrimination specification.
Another problem associated with prior antenna arrays is their size. Prior antenna arrays provided a plurality of radiating elements along the length of the antenna. Therefore, the length of the antenna was dictated by the number and spacing of the radiating elements. Because the gain of an antenna is proportional to the number and spacing of the radiating elements, the width and height of prior antennas could not be reduced significantly without sacrificing antenna gain.
In order to prevent corrosion, there is a need for an antenna capable of preventing water and other environmental elements from impinging upon active antenna components. One solution is providing the antenna with a protective radome. However, one problem with prior antennas is the attachment of the protective radome to the antenna. Because of the manner of attachment of prior radomes, prior radome designs allow water and other environmental elements to impinge upon active antenna components, thereby contributing to antenna corrosion (e.g., the failure of sealants such as caulk). Furthermore, because those prior radomes do not maintain seal integrity over both time and thermal excursions, such radomes allow water and other environmental contaminants to enter the antenna.
Moreover, the visual impact of base station towers on communities has become a societal concern. It has become desirable to reduce the size of these towers and thereby lessen the visual impact of the towers on the community. The size of the towers can be reduced by using base station towers with fewer antennas. This can be achieved if dual polarized antennas and polarization diversity are used. Such systems replace systems using space diversity which requires pairs of vertically polarized antennas. Some studies indicate that, for urban environments, polarization diversity provides signal quality equivalent to space diversity. With the majority of base station sites located in urban environments, it is likely that dual polarized antennas will be used in place of the conventional pairs of vertically polarized antennas. Another way to reduce the size of the base station towers is by using smaller base station antennas. The present invention addresses the problems associated with prior antennas.